A computer cluster generally refers to a group of linked computing devices that work together to perform various computational tasks. The computing devices in the cluster are often connected to each other through local area networks. In many situations, the computer cluster as a whole operates as if it were a single computer. By using multiple computing devices, a cluster can often provide high performance and high availability in a cost-effective manner. Indeed, many of the fastest computers are, in fact, clusters.
Many clusters use some form of shared memory or shared storage to support their operations. However, other varieties of clusters do not have shared memory. These clusters lacking shared memory are called loosely-coupled clusters. Lack of shared memory can make recovery from node failures in the loosely-coupled cluster difficult.